Fontwork is a special kind of a custom shape. You can use it everywhere, where you can use custom shapes. That is in Writer, Calc, Impress and Draw; but not in Math.

If you will make your own, totally new Fontwork shape, you can use a macro and the API, or save a document in flat format (e.g. fodg) and work with an editor on file source. If you really want that, tell us. Than we can give you some pointer.